1. Field of the Invention
The present invention generally relates to tuner calibration, and more specifically to calibrating a local oscillator signal to compensate for variation in the filter passband response of a channel selection filter in a dual conversion tuner.
2. Background Art
Television signals are transmitted at radio frequencies (RF) using terrestrial, cable, or satellite transmission schemes. Terrestrial and cable TV signals are typically transmitted at frequencies of approximately 57 to 860 MHZ, with 6 MHZ channel spacings in the United States and 8 MHz channel spacing in Europe. Satellite TV signals are typically transmitted at frequencies of approximately 980 to 2180 MHz.
Regardless of the transmission scheme, a tuner is utilized to select and down-convert a desired channel from the TV signal to an intermediate frequency (IF) signal or a baseband signal, which is suitable for processing and display on a TV or computer screen. The tuner should provide sufficient image rejection and channel selection during down-conversion as is necessary for the specific application. The National Television Standards Committee (NTSC) sets standards for television signal transmission, reception, and display. To process a NTSC signal, it is preferable that the tuner have a high-level of image rejection. However, less image rejection is acceptable for non-NTSC signals depending on the specific application and the corresponding display requirements.
To achieve a high level of image rejection, traditional TV tuners utilize a dual-conversion architecture having two mixers and at least one surface acoustic wave (SAW) filter. The first mixer up-converts the received RF signal to a first IF frequency (e.g. 1200 MHZ) that is fixed above the RF signal band of the incoming TV signal, using a variable local oscillator (LO) signal. A SAW filter, centered at the first IF, selects the channel of interest and provides the image rejection to prevent signal interference. The second mixer then down-converts the first IF to a lower frequency second IF, using a second fixed frequency LO signal. The second IF output is at baseband for a NTSC compatible signal. Alternatively, the second IF is at 36 or 44 MHZ for a cable system output that is fed into a set-top box or a cable modem. Channel selection is realized by adjusting the first LO signal so that the desired channel is up-converted into the passband of the SAW filter, and is then down-converted to baseband by the second mixer.
The accuracy of the channel selection in the dual conversion tuner is dependent on the accuracy of the passband of the SAW filter. If the passband of the SAW filter varies because of manufacturing tolerances, temperature variations, etc., then the accuracy of the channel selection will suffer. For example, if the passband varies from that intended, then a portion or all of the desired channel may fall outside the SAW passband, causing unwanted signal attenuation in the desired channel.
A conventional method to address the passband tolerance of the SAW filter is to simply increase the passband so as to pass a larger number of channels than is necessary. For instance, SAW filters for TV tuners can be designed to have a passband of 4 or more channels, so as compensate for variation in the passband tolerance. The larger SAW passband improves the likelihood that the desired channel will be up-converted into the SAW filter passband, but also means that one or more undesired channels will also be passed. These unwanted channels can cause signal distortion in the down-conversion stage that requires additional filtering at baseband to correct.
What is needed is a method or apparatus for calibrating the dual conversion tuner (or other type of receiver) for the passband tolerances of the SAW filter, so that the passband can be narrowed to pass approximately only 1 or 2 channels.